#include<iostream>
using namespace std;

int main(){
    ios::sync_with_stdio(0),cin.tie(0),cout.tie(0);
    
    int a, b, f, k;
    
    cin >> a >> b >> f >> k;
    if(b < f){
        cout << -1 << endl;
        return 0;
    }else{
        int df = b - f;
        int ans = 0;
        
        for(int i = 1; i < k; i++){
            if(i & 1){
                if(df < 2 * (a - f)){
                    ans++;
                    df = b;
                }
                df -= 2 * (a - f);
            }
            else{
                if(df < 2 * f){
                    ans++;
                    df = b;
                }
                df -= 2 * f;
            }
            if(df < 0){
                cout << -1 << endl;
                return 0;
            }
        }
        if(k & 1){
            if(df < a - f){
                ans++;
                df = b;
            }
            if(df < a - f){
                ans = -1;
            }
        }
        else{
            if(df < f){
                ans++;
                df = b;
            }
        }
        cout << ans << endl;
    }
    return 0;
} 
